1. Field of the Invention
The present invention relates to an electrodeposition method for depositing oxide, particularly zinc oxide on a substrate by electrodeposition (for example electrolytic plating or electrolytic deposition), and more particularly to an electrodeposition method enabling stable deposition of oxide with satisfactory productivity.
2. Related Background Art
For depositing a functional film, there are known various methods such as resistance heating evaporation, CVD, sputtering, spray pyrolysis and electroplating.
Among these methods, the electroplating method (same as xe2x80x9cplatingxe2x80x9d and included in the wet process), in which a material dissolved in aqueous solution is electrochemically deposited on a substrate, has the following advantages and is applicable to an elongated or longitudinal substrate.
The elongated or longitudinal substrate means a substrate of an extremely oblong rectangular shape that can be wound longitudinally in a roll shape and may be called in various names such as rolled substrate, web, hoop, coil, tape, reeled material etc. but is hereinafter called the longitudinal substrate.
Such longitudinal substrate enables continuous film formation and is industrially extremely advantageous in elevating the work rate of the apparatus or lowering the running cost thereof.
A first advantage in the electroplating is that the film formation can be made extremely simple, for example in contrast to the vacuum apparatus employed in the sputtering process. More specifically, the electroplating can dispense with the expensive vacuum pump, and the film deposition can be achieved in an extremely simple manner as the designing of the power source and the electrode required for utilizing plasma is not required.
A second advantage in the electroplating is that the running cost is generally low. In the sputtering process, the running cost is high because the preparation of the target material is costly, requiring manpower and facility therefor, and the efficiency of use of the target is limited to about 20%. Also in case where the throughput of the film depositing apparatus has to be elevated or in case where a large film thickness is required, the target replacing operation occupies a considerable weight so that the work efficiency becomes inevitably low.
In terms of the running cost, the electroplating method is also superior to other methods such as CVD or vacuum evaporation.
A third advantage in the electroplating method is the superiority in the conductive and optical characteristics. The film formed on the longitudinal substrate is usually composed of polycrystalline fine particles, and the film obtained by electroplating is comparable in the conductive and optical characteristics to the film obtained by the vacuum method and is superior to the films obtained by sol-gel process, coating process utilizing organic substances or spray pyrolysis process.
A fourth advantage of the electroplating lies in a fact that the above-described advantages can be obtained even in case of forming an oxide, and that the waste liquid can be easily processed whereby the influence on the environment is limited and the cost for preventing the environmental pollution is low.
Various electroplating apparatus capable of film formation on the longitudinal substrate are already known, and an example of such apparatus is provided with rollers at the entrance and exit sides of an electroplating tank for holding and transporting a longitudinal substrate thereby continuously passing the substrate in the electroplating tank and an electric current is passed between the rollers and an anode (counter electrode) provided in the electroplating tank so as to be positioned under the longitudinal substrate, thereby forming a film on the surface of the longitudinal substrate.
However, the above-described conventional electroplating apparatus has been associated with the following drawbacks.
For example, in case of employing a longitudinal substrate having a resistance of about 0.01xcexa9 per meter and an electroplating current in the order of several tens of Amperes, there is generated an enormous heat loss. For this reason, there is generated peeling of the film or stain on the surface of the plated zinc oxide film, presumably induced by heat, thereby resulting in non-uniform film formation.
Also as the position of current feeding becomes distant from the electroplating bath, the power supply voltage drops whereby the control for uniform film formation becomes more difficult.
In consideration of the foregoing, the object of the present invention is to provide an electrodeposition method capable of suppressing the drop in the power supply voltage and minimizing the heat loss caused by the electrodeposition current, thereby achieving uniform film formation with satisfactory characteristics.
The above-mentioned object can be attained, according to the present invention, by an electrodeposition method of dipping a conductive substrate in an electrodeposition tank holding an electrodeposition bath and electrolytically depositing an oxide on the conductive substrate, which comprises using, as at least one electrode, electricity feed means comprising a conductive member so provided as to be in contact with the conductive substrate, wherein the contact position of the electricity feed means and the conductive substrate is outside the electrodeposition bath, and wherein the resistance, including contact resistance, between the closer to the electricity feed means of the position of entry of the conductive substrate into the electrodeposition bath and the position of discharge of the conductive substrate from the electrodeposition bath and the contact position of the conductive substrate with the electricity feed means is 20xcexa9 or less.
Also the feed means is preferably shaped as a roller.
Also there are preferably provided at least two electrodeposition tanks.
It is also preferred that at least a turning back means contacting the front surface or the back surface of the conductive substrate is provided between the electrodeposition tanks and between at least two rollers in contact with the back surface of the conductive substrate, and that the contact position between the electricity feed means and the conductive substrate is between the above-mentioned rollers.
Also the turning back means is preferably shaped as a roller.
Also the turning back means preferably serves also as the electricity feed means.